1. Field of the Invention
The present invention relates to methods for manufacturing epitaxial layers suitable for substrates included in semiconductor devices such as photonic devices and electronic devices. The present invention particularly relates to a method for manufacturing a nitride film including a high-resistivity GaN layer having a resistivity of 104 Ω·cm or more and also relates to an epitaxial substrate manufactured by the method.
2. Description of the Related Art
Films containing Group-III nitrides such as GaN are used as semiconductor films included in semiconductor devices such as photonic devices and electronic devices. Such films are also used as semiconductor films included in high-speed IC chips; hence, the films have been attracting much attention in recent years. In order to form a layer containing a Group-III nitride, the following method has been developed and is now in practical use: the nitride layer is epitaxially grown on a monocrystalline wafer, made of sapphire or the like, by a vapor reaction process or the like.
Described below is a known technique for epitaxially growing the Group-III nitride layer by a vapor reaction process.
A predetermined monocrystalline wafer is provided on a susceptor placed in a reaction chamber and is then heated to a predetermined temperature with heaters arranged inside and outside the susceptor. A gaseous organic metal compound containing a Group-III element and a gaseous nitrogen compound are fed to the reaction tube together with a carrier gas, and a gaseous compound containing another element is also fed thereto according to needs. The gas mixture is allowed to flow over the wafer, whereby a group of layers containing a single Group-III nitride or a plurality of Group-III nitrides are formed by a chemical vapor reaction process.
In the Group-III nitride layers, a GaN layer can be applied to an electronic device such as a field effect transistor if the GaN layer is combined with, for example, a mixed crystal layer containing a Group-III nitride such as AlGaN so as to form a heterostructure.
Since electronic devices having an AlGaN/GaN high-electron-mobility transistor (HEMT) structure disclosed in Khan et al., Appl. Phys. Lett., 63 (1993), 1214 have been achieved, the development thereof is being conducted all over the world. Those GaN electronic devices have been prepared by epitaxially growing predetermined semiconductor layers on sapphire wafers.
FIG. 1 is a schematic view showing a configuration of a field effect transistor functioning as a semiconductor device including a GaN layer.
The field effect transistor 10 shown in FIG. 1 includes a monocrystalline wafer 1 made of monocrystalline sapphire; a base layer 2, grown at about 500° C., containing GaN; a buffer layer 3, grown at 1,000° C. or more, containing GaN; a spacer layer 4 containing AlGaN; a carrier supply layer 5, containing Si-doped n-AlGaN; and a barrier layer 6 containing AlGaN, those layers being disposed on the monocrystalline wafer 1 in that order.
The surface of the barrier layer 6 are partially exposed. The following electrodes are disposed on the respective exposed portions: a source electrode 7 and a drain electrode 8, made of , for example, Ti/Al, which corresponds to an ohmic contact metal for n-type semiconductor layer and a gate electrode 9, made of, for example, Pt/Au, which corresponds to a Schottky contact metal for n-type semiconductor layer.
Electrons generated from the carrier supply layer 5 are confined at the heterointerface between the buffer layer 3 and the spacer layer 4. Therefore, when a predetermined voltage is applied between the source electrode 7 and the drain electrode 8, the electrons are allowed to flow at the interface, whereby current flows in the field effect transistor 10. In order to impart good conduction properties to the field effect transistor, it is critical to form an ideal heterointerface structure that rarely contains impurities and has crystal defects.
On the other hand, in order to allow current interruption to occur in the field effect transistor 10, a predetermined voltage is applied between the gate electrode 9 and the drain electrode 8 while a predetermined voltage is being applied between the source electrode 7 and the drain electrode 8. That is, a reverse bias is applied to the Schottky junction, thereby a depletion region is formed directly under the gate electrode 9 to interrupt the current.
In a field effect transistor, as is the above transistor, including a GaN layer functioning as a buffer layer, the GaN layer must have sufficiently high resistivity. This is because, in the operation of the field effect transistor, current leakage occurs during current interruption if the GaN layer does not have sufficiently high resistivity. Therefore, the GaN layer preferably has a resistivity of 104 Ω·cm or more, more preferably 105 Ω·cm or more, and further more preferably 106 Ω·cm or more.
However, when the GaN layer is grown by the chemical vapor deposition method described above, residual donors of which the number is about 1×1016 per cubic centimeter and which have an electrically active level remain in the GaN layer even if the GaN layer is not intentionally doped with impurities. Therefore, it has not been able to prepare a GaN layer having a resistivity of 10 Ω·cm or more.
On the other hand, the following technique has been proposed: a GaN layer is increased in resistivity by doping the GaN layer with, for example, an acceptor impurity such as Mg during the growth of the GaN layer or implanting ions into the GaN layer after the layer is formed. However, for electronic devices, it is not preferable that the GaN layer contain any intentionally added impurity. Furthermore, an ideal heterointerface cannot be achieved because the impurity may migrate to the heterointerface. Therefore, desired conduction properties cannot be obtained in some cases.